A related patent application, also entitled “Closure Assembly for Electrochemical Cells” and having U.S. patent application Ser. No. 12/391,742, was filed on the same day as this application and is hereby incorporated by reference.
Electrochemical cells, including but not limited to those with a lithium metal or alloy as an electrochemically active material, often utilize one or more positive temperature coefficient (“PTC”) safety devices. These devices limit the current that can flow through the cell in order under certain conditions. For example, excess heat sufficient to activate the PTC device may be generated in an electrochemical cell as a result of external short circuit, attempting to recharge a primary cell, improperly charging a rechargeable cell, forced overdischarge, or improper installation of cells in a device.
Typically, PTC devices include a layer comprising a polymer and conductive particles such as carbon. When the temperature of the PTC device is increased above an activating temperature, the polymer thermally expands in a way that electrically disconnects the conductive particles dispersed within the PTC, thereby cutting off the flow of current through the PTC device. Consequently, electrochemical cell designs must allow for the thermal expansion of the PTC device.
Cylindrical electrochemical cells, such as AA and AAA sized batteries, are formed by a can (i.e., a cylinder with a closed bottom) and a cover and have an overall can height that is larger than the can's diameter. The electrical terminals of the battery are integrally formed on the bottom of the can and the cover. The container (i.e., the combination of the can and the cover) is then sealed by compressing a gasket or seal member between the cover and a portion of the open end of the can. In order to insure a hermetic seal, the compressive force should be maintained in both the axial and radial directions of the cylinder, usually by beading the can's sidewalls and then crimping the edge of the open end of the can over the cover. Insofar as the PTC device is often connected to the cover, this closure may subject the PTC device to compressive axial forces that adversely affect the activation of the PTC.
A common closure used in commercially available lithium-iron disulfide cells is shown in FIG. 7. Electrochemical cell 1 includes a cover 2 and PTC device 4 configured at a terminal end of the cell. Gasket 6 has an axial middle portion with a substantially uniform shape. The cover 2, PTC device 4 and contact assembly 8 (which includes both a rollback cover and a spring) are held, housed, or retained within the C-shaped gasket 6. Notably, axial force must be exerted to crimp the terminal edge of the can 3 during the sealing of the cell, thereby exposing the PTC device 4 to axial compressive forces during the closing operation itself. Moreover, because the crimped edge remains in place and the elastomeric gasket remains axially compressed, it will continue to axially constrict activation of the PTC (which requires the axial expansion of the PTC) throughout the life of the battery.
Various approaches have attempted to allow the PTC device to remove the PTC device from unwanted axially compression, thereby allowing it to expand upon activation. One such approach contemplates the use of additional conductive members, and/or spring-like devices, although this requires a substantial reconfiguration (and reduction in size) of the PTC device. Gasket materials that softens at a temperature below the activation of the PTC device have also been used, but this may eliminate the use of the best performing materials. Yet another approach is to locate the PTC outside of the container, but this requires a means for attaching the PTC to the can/cover and increases the likelihood of damage to the PTC.
U.S. Pat. No. 6,090,322 relates to a method for integrally insert-molding a C-shaped gasket onto a peripheral edge of a metal sealing plate in a coin cell. U.S. Pat. No. 6,274,267 describes insert-molding a question-marked shaped gasket around a cylindrical cell cover plate. In some embodiments of this latter patent, a PTC device is located between a bead and a crimped portion of the open end of the container, adjacent one or more of a terminal cover; however, this arrangement subjects the PTC device to compressive axial forces during sealing of the cell.
U.S. Pat. No. 5,376,467 describes an organic electrolyte battery having a positive temperature coefficient resistor. In one embodiment, the PTC resistor is carried on a conductive annular member so that the PTC resistor is spaced radially inward, away from a crimping zone. In a second embodiment, it is disposed in the center of the lid and connected to the sealing member by a support member. In both instances, these arrangements necessarily require welding or adhesively fixing the PTC resistor to an additional conductive sealing member and the PTC resistor must have a diameter that is substantially smaller than the inner diameter of the battery can, thereby limiting the amount of surface area and overall effectiveness of the PCT resistor.
U.S. Pat. No. 5,766,790 relates to a safety device for use in a secondary battery that relies upon a series of disk-shaped springs. Internal pressure from the battery housing deforms the springs so as to break the electrical contact between the external terminal and one of the disk-shaped springs. Notably, this device requires numerous moving parts and relies solely on the internal pressure within the cell caused by overheating the electrolyte, rather than being activated by the electrical demands (i.e., the load) placed on the cell.
U.S. Pat. No. 6,531,242 and Japanese Publication No. 05-151944 disclose the use of multiple gaskets in a battery seal. These gaskets work together to minimize the compressive forces exerted on the PTC device. In the former, a series of nested gaskets cooperate in conjunction with a lead plate and the PTC device. In the later, two separate gaskets are provided, with the gasket that comes into contact with the PTC device having a lower melting point than the activation temperature of the PTC device, thereby insuring the PTC device can expand as necessary into the softened gasket. The inclusion of additional parts (e.g., two or more gaskets) increases manufacturing complexity and cost.
U.S. Pat. No. 6,620,544 discloses and electrochemical cell that relies upon a metal foam “shock absorber” and a separate insulating ring both positioned proximate to the PTC device. Here, the metal foam allows for expansion of the PTC device upon activation, while the insulating ring is thicker than the PTC device to allow for proper spacing of the parts when the cell is sealed. As with U.S. Pat. No. 5,376,467 above, this arrangement requires the use of a smaller diameter PTC device.
Finally, Japanese Publication No. 10-162805 contemplates providing a PTC device along the central axis of the cell. Here, the PTC device avoids exposure to crimping forces by limiting its overall diameter, although this limited diameter reduces the effectiveness of the PTC device by limiting the amount of surface area in contact with the electrode. Moreover, this central location of the PTC device prevents the inclusion of common venting devices. Finally, as noted in the reference, some embodiments of this arrangement permit the PTC device to be in contact with the organic electrolyte contained within the cell housing. In such instances, the PTC device must not react with or dissolve in the organic solvents, thereby presenting a significant technical challenge in terms of chemical compatibility.